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        </div><article class="page single"><h1 class="single-title animated flipInX">Spring源码 - web容器启动时ServletContainerInitializer的加载</h1><div class="post-meta">
            <div class="post-meta-line"><span class="post-author"><a href="https://wlizhi.cc" title="Author" target="_blank" rel="noopener noreffer author" class="author"><i class="fas fa-user-circle fa-fw"></i>Wlizhi</a></span>&nbsp;<span class="post-category">收录于 <a href="/categories/spring/"><i class="far fa-folder fa-fw"></i>spring</a></span></div>
            <div class="post-meta-line"><i class="far fa-calendar-alt fa-fw"></i>&nbsp;<time datetime="2020-12-10">2020-12-10</time>&nbsp;<i class="fas fa-pencil-alt fa-fw"></i>&nbsp;约 3317 字&nbsp;
                <i class="far fa-clock fa-fw"></i>&nbsp;预计阅读 7 分钟&nbsp;<span id="/posts/spring/21-load-servlet-container-initializer-on-startup/" class="leancloud_visitors" data-flag-title="Spring源码 - web容器启动时ServletContainerInitializer的加载">
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            </div><div class="content" id="content"><p>spring源码系列文章，示例代码的中文注释，均是 copy 自 <a href="https://gitee.com/wlizhi/spring-framework" target="_blank" rel="noopener noreffer">https://gitee.com/wlizhi/spring-framework</a> 。</p>
<p>链接中源码是作者从 github 下载，并以自身理解对核心流程及主要节点做了详细的中文注释。</p>
<hr>
<p>在web容器启动时为提供给第三方组件机会做一些初始化的工作，例如注册 servlet 或者 filter 等，servlet 规范中通过 ServletContainerInitializer 实现此功能。</p>
<p>伴随着 ServletContainerInitializer 一起起作用的还有 @HandlesTypes 注解，通过这个注解会将指定的类注入到 ServletContainerInitializer 的 onStartup 方法参数中。</p>
<!--more-->
<p>ServletContainerInitializer 是通过 SPI 的方式扫描加载的，在对应 jar 包的 META-INF/services 下，会扫描 javax.servlet.ServletContainerInitializer 文件，加载文件中定义的实现类。</p>
<p>SpringMvc 就是利用这种方式，完成 DispatcherServlet 的初始化工作。</p>
<p>在 spring-web 包 META-INF/services 下就有 javax.servlet.ServletContainerInitializer 文件，文件中是这么定义的：</p>
<p><code>org.springframework.web.SpringServletContainerInitializer</code></p>
<p>SpringServletContainerInitializer 源码如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="nd">@HandlesTypes</span><span class="o">(</span><span class="n">WebApplicationInitializer</span><span class="o">.</span><span class="na">class</span><span class="o">)</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">SpringServletContainerInitializer</span> <span class="kd">implements</span> <span class="n">ServletContainerInitializer</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">	<span class="nd">@Override</span>
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="kt">void</span> <span class="nf">onStartup</span><span class="o">(</span><span class="nd">@Nullable</span> <span class="n">Set</span><span class="o">&lt;</span><span class="n">Class</span><span class="o">&lt;?&gt;&gt;</span> <span class="n">webAppInitializerClasses</span><span class="o">,</span> <span class="n">ServletContext</span> <span class="n">servletContext</span><span class="o">)</span>
</span></span><span class="line"><span class="cl">			<span class="kd">throws</span> <span class="n">ServletException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 根据servlet规范，tomcat会在启动的时候，以SPI方式加载 services/javax.servlet.ServletContainerInitializer，并调用onStartup方法。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="c1">// 而spring-web包下就定义了此类。所以服务器启动时，会执行到这里。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 这个列表封装了所有SPI加载的 WebApplicationInitializer 实例。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">List</span><span class="o">&lt;</span><span class="n">WebApplicationInitializer</span><span class="o">&gt;</span> <span class="n">initializers</span> <span class="o">=</span> <span class="k">new</span> <span class="n">LinkedList</span><span class="o">&lt;&gt;();</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">webAppInitializerClasses</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">for</span> <span class="o">(</span><span class="n">Class</span><span class="o">&lt;?&gt;</span> <span class="n">waiClass</span> <span class="o">:</span> <span class="n">webAppInitializerClasses</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// Be defensive: Some servlet containers provide us with invalid classes,
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="c1">// no matter what @HandlesTypes says...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="c1">// 非接口、非抽象类，且是WebApplicationInitializer实现类的，将会反射创建对象。封装到上面定义的列表中。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="k">if</span> <span class="o">(!</span><span class="n">waiClass</span><span class="o">.</span><span class="na">isInterface</span><span class="o">()</span> <span class="o">&amp;&amp;</span> <span class="o">!</span><span class="n">Modifier</span><span class="o">.</span><span class="na">isAbstract</span><span class="o">(</span><span class="n">waiClass</span><span class="o">.</span><span class="na">getModifiers</span><span class="o">())</span> <span class="o">&amp;&amp;</span>
</span></span><span class="line"><span class="cl">						<span class="n">WebApplicationInitializer</span><span class="o">.</span><span class="na">class</span><span class="o">.</span><span class="na">isAssignableFrom</span><span class="o">(</span><span class="n">waiClass</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="k">try</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">initializers</span><span class="o">.</span><span class="na">add</span><span class="o">((</span><span class="n">WebApplicationInitializer</span><span class="o">)</span>
</span></span><span class="line"><span class="cl">								<span class="n">ReflectionUtils</span><span class="o">.</span><span class="na">accessibleConstructor</span><span class="o">(</span><span class="n">waiClass</span><span class="o">).</span><span class="na">newInstance</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="k">catch</span> <span class="o">(</span><span class="n">Throwable</span> <span class="n">ex</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="k">throw</span> <span class="k">new</span> <span class="n">ServletException</span><span class="o">(</span><span class="s">&#34;Failed to instantiate WebApplicationInitializer class&#34;</span><span class="o">,</span> <span class="n">ex</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="c1">// 排序，按照排序后的顺序，遍历、调用onStartup()。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">servletContext</span><span class="o">.</span><span class="na">log</span><span class="o">(</span><span class="n">initializers</span><span class="o">.</span><span class="na">size</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34; Spring WebApplicationInitializers detected on classpath&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="n">AnnotationAwareOrderComparator</span><span class="o">.</span><span class="na">sort</span><span class="o">(</span><span class="n">initializers</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="k">for</span> <span class="o">(</span><span class="n">WebApplicationInitializer</span> <span class="n">initializer</span> <span class="o">:</span> <span class="n">initializers</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="n">initializer</span><span class="o">.</span><span class="na">onStartup</span><span class="o">(</span><span class="n">servletContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>SpringServletContainerInitializer 类声明处定义了 <code>@HandlesTypes(WebApplicationInitializer.class)</code>，web容器启动时，会调用 SpringServletContainerInitializer 的 onStartup()，并将项目中存在的所有 WebApplicationInitializer 实现类的 Class 对象，作为参数传递给 onStartup()。</p>
<p>遍历这个集合，将所有的实现类通过反射调用无参构造函数进行实例化。实例化后，得到一个实例列表，给这个列表中实例进行排序后，依次调用每个实例的 onStartup()。</p>
<p>我们可以实现 WebApplicationInitializer，或者继承它的子类，通过实现一些特定的方法，来达到在容器启动时，加载一些内容的目的。</p>
<p>首先看已经实现了一些方法的子类。以下是 AbstractDispatcherServletInitializer 中 onStartup() 源码：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractDispatcherServletInitializer</span> <span class="kd">extends</span> <span class="n">AbstractContextLoaderInitializer</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">	<span class="nd">@Override</span>
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="kt">void</span> <span class="nf">onStartup</span><span class="o">(</span><span class="n">ServletContext</span> <span class="n">servletContext</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">ServletException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 创建 RootApplicationContext，也就是spring父容器。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="c1">// 后面的流程中，设置子容器的父容器时，就是通过servletContext作为桥梁，进行获取的。父容器实例化后，会注册到servletContext。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="kd">super</span><span class="o">.</span><span class="na">onStartup</span><span class="o">(</span><span class="n">servletContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="n">registerDispatcherServlet</span><span class="o">(</span><span class="n">servletContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>主要调用了两个方法 <code>super.onStartup()</code> 和 <code>registerDispatcherServlet()</code>。</p>
<hr>
<p>super.onStartup() 源码中调用了 registerContextLoaderListener()，源码如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractContextLoaderInitializer</span> <span class="kd">implements</span> <span class="n">WebApplicationInitializer</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="kt">void</span> <span class="nf">registerContextLoaderListener</span><span class="o">(</span><span class="n">ServletContext</span> <span class="n">servletContext</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 创建 RootApplicationContext，这里面会有一个回调 getRootConfigClasses。这个回调中获取了实现类方法中返回的配置类。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">WebApplicationContext</span> <span class="n">rootAppContext</span> <span class="o">=</span> <span class="n">createRootApplicationContext</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">rootAppContext</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 创建监听器，监听器中封装了ApplicationContext，将这个监听器封装到servletContext中
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="n">ContextLoaderListener</span> <span class="n">listener</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ContextLoaderListener</span><span class="o">(</span><span class="n">rootAppContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="n">listener</span><span class="o">.</span><span class="na">setContextInitializers</span><span class="o">(</span><span class="n">getRootApplicationContextInitializers</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">			<span class="n">servletContext</span><span class="o">.</span><span class="na">addListener</span><span class="o">(</span><span class="n">listener</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>创建了一个 ApplicationContext，这是spring父容器，后面会创建一个子容器。将其放到一个 ServletContextListener 监听器中。</p>
<p>创建 ApplicationContext 源码：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractAnnotationConfigDispatcherServletInitializer</span>
</span></span><span class="line"><span class="cl">		<span class="kd">extends</span> <span class="n">AbstractDispatcherServletInitializer</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="n">WebApplicationContext</span> <span class="nf">createRootApplicationContext</span><span class="o">()</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 这里获取到具体实现类中此方法返回的配置类型。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">Class</span><span class="o">&lt;?&gt;[]</span> <span class="n">configClasses</span> <span class="o">=</span> <span class="n">getRootConfigClasses</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(!</span><span class="n">ObjectUtils</span><span class="o">.</span><span class="na">isEmpty</span><span class="o">(</span><span class="n">configClasses</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 创建上下文对象，并将配置类注入到上下文对象中。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="n">AnnotationConfigWebApplicationContext</span> <span class="n">context</span> <span class="o">=</span> <span class="k">new</span> <span class="n">AnnotationConfigWebApplicationContext</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">			<span class="n">context</span><span class="o">.</span><span class="na">register</span><span class="o">(</span><span class="n">configClasses</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="k">return</span> <span class="n">context</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">return</span> <span class="kc">null</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>这里调用了 getRootConfigClasses()，这是一个模板方法，如果我们通过继承这个类的方式，来重写这个方法，就可以将容器启动的配置类，加载到spring父容器中。</p>
<p>监听器的 contextInitialized() 方法中，会调用 initWebApplicationContext()。</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">ContextLoader</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="n">WebApplicationContext</span> <span class="nf">initWebApplicationContext</span><span class="o">(</span><span class="n">ServletContext</span> <span class="n">servletContext</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">servletContext</span><span class="o">.</span><span class="na">log</span><span class="o">(</span><span class="s">&#34;Initializing Spring root WebApplicationContext&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="n">Log</span> <span class="n">logger</span> <span class="o">=</span> <span class="n">LogFactory</span><span class="o">.</span><span class="na">getLog</span><span class="o">(</span><span class="n">ContextLoader</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="k">if</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">context</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="k">this</span><span class="o">.</span><span class="na">context</span> <span class="o">=</span> <span class="n">createWebApplicationContext</span><span class="o">(</span><span class="n">servletContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="k">if</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">context</span> <span class="k">instanceof</span> <span class="n">ConfigurableWebApplicationContext</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="n">ConfigurableWebApplicationContext</span> <span class="n">cwac</span> <span class="o">=</span> <span class="o">(</span><span class="n">ConfigurableWebApplicationContext</span><span class="o">)</span> <span class="k">this</span><span class="o">.</span><span class="na">context</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">            <span class="k">if</span> <span class="o">(!</span><span class="n">cwac</span><span class="o">.</span><span class="na">isActive</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">                <span class="c1">// The context has not yet been refreshed -&gt; provide services such as
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                <span class="c1">// setting the parent context, setting the application context id, etc
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                <span class="k">if</span> <span class="o">(</span><span class="n">cwac</span><span class="o">.</span><span class="na">getParent</span><span class="o">()</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">                    <span class="c1">// The context instance was injected without an explicit parent -&gt;
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                    <span class="c1">// determine parent for root web application context, if any.
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                    <span class="n">ApplicationContext</span> <span class="n">parent</span> <span class="o">=</span> <span class="n">loadParentContext</span><span class="o">(</span><span class="n">servletContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">                    <span class="n">cwac</span><span class="o">.</span><span class="na">setParent</span><span class="o">(</span><span class="n">parent</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">                <span class="o">}</span>
</span></span><span class="line"><span class="cl">                <span class="c1">// TODO 调用了 refresh()
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>                <span class="n">configureAndRefreshWebApplicationContext</span><span class="o">(</span><span class="n">cwac</span><span class="o">,</span> <span class="n">servletContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">            <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// 将WebApplicationContext添加到servletContext中，后面子容器设置父容器时，就会通过这个键值进行获取。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="n">servletContext</span><span class="o">.</span><span class="na">setAttribute</span><span class="o">(</span><span class="n">WebApplicationContext</span><span class="o">.</span><span class="na">ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE</span><span class="o">,</span> <span class="k">this</span><span class="o">.</span><span class="na">context</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">        <span class="n">ClassLoader</span> <span class="n">ccl</span> <span class="o">=</span> <span class="n">Thread</span><span class="o">.</span><span class="na">currentThread</span><span class="o">().</span><span class="na">getContextClassLoader</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">        <span class="k">if</span> <span class="o">(</span><span class="n">ccl</span> <span class="o">==</span> <span class="n">ContextLoader</span><span class="o">.</span><span class="na">class</span><span class="o">.</span><span class="na">getClassLoader</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="n">currentContext</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">context</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="k">else</span> <span class="k">if</span> <span class="o">(</span><span class="n">ccl</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="n">currentContextPerThread</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="n">ccl</span><span class="o">,</span> <span class="k">this</span><span class="o">.</span><span class="na">context</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// 省略...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="k">return</span> <span class="k">this</span><span class="o">.</span><span class="na">context</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>这里重点有两个</p>
<ol>
<li>
<p>configureAndRefreshWebApplicationContext() 调用了 refresh() 方法，刷新容器。这时容器中已经封装了定义的配置类。会根据配置类进行递归扫描。</p>
</li>
<li>
<p>servletContext.setAttribute()，将这个 ApplicationContext 保存到 ServletContext 中，这样，在子容器创建后，设置父容器，就可以从 ServletContext 中获取。</p>
</li>
</ol>
<hr>
<p>回到 onStartup()，在调用完 super.onStartup()之后，会调用 registerDispatcherServlet()，源码如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractDispatcherServletInitializer</span> <span class="kd">extends</span> <span class="n">AbstractContextLoaderInitializer</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="kt">void</span> <span class="nf">registerDispatcherServlet</span><span class="o">(</span><span class="n">ServletContext</span> <span class="n">servletContext</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">String</span> <span class="n">servletName</span> <span class="o">=</span> <span class="n">getServletName</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">		<span class="n">Assert</span><span class="o">.</span><span class="na">hasLength</span><span class="o">(</span><span class="n">servletName</span><span class="o">,</span> <span class="s">&#34;getServletName() must not return null or empty&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 创建spring子容器。这里面通过回调方法 getServletConfigClasses。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">WebApplicationContext</span> <span class="n">servletAppContext</span> <span class="o">=</span> <span class="n">createServletApplicationContext</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">		<span class="n">Assert</span><span class="o">.</span><span class="na">notNull</span><span class="o">(</span><span class="n">servletAppContext</span><span class="o">,</span> <span class="s">&#34;createServletApplicationContext() must not return null&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 创建 DispatcherServlet。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">FrameworkServlet</span> <span class="n">dispatcherServlet</span> <span class="o">=</span> <span class="n">createDispatcherServlet</span><span class="o">(</span><span class="n">servletAppContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="n">Assert</span><span class="o">.</span><span class="na">notNull</span><span class="o">(</span><span class="n">dispatcherServlet</span><span class="o">,</span> <span class="s">&#34;createDispatcherServlet(WebApplicationContext) must not return null&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="n">dispatcherServlet</span><span class="o">.</span><span class="na">setContextInitializers</span><span class="o">(</span><span class="n">getServletApplicationContextInitializers</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 将servlet添加到servletContext中
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">ServletRegistration</span><span class="o">.</span><span class="na">Dynamic</span> <span class="n">registration</span> <span class="o">=</span> <span class="n">servletContext</span><span class="o">.</span><span class="na">addServlet</span><span class="o">(</span><span class="n">servletName</span><span class="o">,</span> <span class="n">dispatcherServlet</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">registration</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">throw</span> <span class="k">new</span> <span class="n">IllegalStateException</span><span class="o">(</span><span class="s">&#34;Failed to register servlet with name &#39;&#34;</span> <span class="o">+</span> <span class="n">servletName</span> <span class="o">+</span> <span class="s">&#34;&#39;. &#34;</span> <span class="o">+</span>
</span></span><span class="line"><span class="cl">					<span class="s">&#34;Check if there is another servlet registered under the same name.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="c1">// 如果该元素的值为负数或者没有设置，则容器会当Servlet被请求时再加载。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="c1">// 如果值为正整数或者0时，表示容器在应用启动时就加载并初始化这个servlet，
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="c1">// 值越小，servlet的优先级越高，就越先被加载
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">registration</span><span class="o">.</span><span class="na">setLoadOnStartup</span><span class="o">(</span><span class="n">1</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="n">registration</span><span class="o">.</span><span class="na">addMapping</span><span class="o">(</span><span class="n">getServletMappings</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">		<span class="n">registration</span><span class="o">.</span><span class="na">setAsyncSupported</span><span class="o">(</span><span class="n">isAsyncSupported</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="n">Filter</span><span class="o">[]</span> <span class="n">filters</span> <span class="o">=</span> <span class="n">getServletFilters</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(!</span><span class="n">ObjectUtils</span><span class="o">.</span><span class="na">isEmpty</span><span class="o">(</span><span class="n">filters</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">for</span> <span class="o">(</span><span class="n">Filter</span> <span class="n">filter</span> <span class="o">:</span> <span class="n">filters</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="n">registerServletFilter</span><span class="o">(</span><span class="n">servletContext</span><span class="o">,</span> <span class="n">filter</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="n">customizeRegistration</span><span class="o">(</span><span class="n">registration</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>这里创建了子容器 WebApplicationContext 和 DispatcherServlet，子容器就保存在 DispatcherServlet 中。</p>
<p>DispatcherServlet.init() -&gt; initServletBean() -&gt; initWebApplicationContext()</p>
<p>initWebApplicationContext() 源码：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">FrameworkServlet</span> <span class="kd">extends</span> <span class="n">HttpServletBean</span> <span class="kd">implements</span> <span class="n">ApplicationContextAware</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="n">WebApplicationContext</span> <span class="nf">initWebApplicationContext</span><span class="o">()</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">WebApplicationContext</span> <span class="n">rootContext</span> <span class="o">=</span>
</span></span><span class="line"><span class="cl">				<span class="n">WebApplicationContextUtils</span><span class="o">.</span><span class="na">getWebApplicationContext</span><span class="o">(</span><span class="n">getServletContext</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">		<span class="n">WebApplicationContext</span> <span class="n">wac</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="c1">// TODO 注解方式激活MVC功能时，会走到这里
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="k">if</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">webApplicationContext</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// A context instance was injected at construction time -&gt; use it
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="n">wac</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">webApplicationContext</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">			<span class="k">if</span> <span class="o">(</span><span class="n">wac</span> <span class="k">instanceof</span> <span class="n">ConfigurableWebApplicationContext</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="n">ConfigurableWebApplicationContext</span> <span class="n">cwac</span> <span class="o">=</span> <span class="o">(</span><span class="n">ConfigurableWebApplicationContext</span><span class="o">)</span> <span class="n">wac</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">				<span class="k">if</span> <span class="o">(!</span><span class="n">cwac</span><span class="o">.</span><span class="na">isActive</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 设置父容器，这个父容器时从servletContext中获取的。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="c1">// 父容器是在 ContextLoader.initWebApplicationContext()，调用完refresh()后设置到servletContext中的。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="k">if</span> <span class="o">(</span><span class="n">cwac</span><span class="o">.</span><span class="na">getParent</span><span class="o">()</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// The context instance was injected without an explicit parent -&gt; set
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="c1">// the root application context (if any; may be null) as the parent
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="n">cwac</span><span class="o">.</span><span class="na">setParent</span><span class="o">(</span><span class="n">rootContext</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// TODO 配置、刷新WebApplicationContext，调用DispatcherServlet的init()
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="n">configureAndRefreshWebApplicationContext</span><span class="o">(</span><span class="n">cwac</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 省略...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(!</span><span class="k">this</span><span class="o">.</span><span class="na">refreshEventReceived</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="kd">synchronized</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">onRefreshMonitor</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// 初始化 initHandlerMappings、initHandlerAdapters 等
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="n">onRefresh</span><span class="o">(</span><span class="n">wac</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">    
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">publishContext</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// Publish the context as a servlet context attribute.
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="n">String</span> <span class="n">attrName</span> <span class="o">=</span> <span class="n">getServletContextAttributeName</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">			<span class="n">getServletContext</span><span class="o">().</span><span class="na">setAttribute</span><span class="o">(</span><span class="n">attrName</span><span class="o">,</span> <span class="n">wac</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">wac</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>首先获取 rootContext，然后获取到 DispatcherServlet 中已经封装的 WebApplicationContext，将 rootContext 设置为 WebApplicationContext 的父容器。然后调用 configureAndRefreshWebApplicationContext()，进而调用子容器的 refresh()，完成上下文的加载。</p>
<p>最后 onRefresh() 中完成了 DispatcherServlet 中很多必要组件的初始化，包括 initHandlerMappings、initHandlerAdapters 等。</p>
<p>获取父容器会调用到这里：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">WebApplicationContextUtils</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="kd">public</span> <span class="kd">static</span> <span class="n">WebApplicationContext</span> <span class="nf">getWebApplicationContext</span><span class="o">(</span><span class="n">ServletContext</span> <span class="n">sc</span><span class="o">,</span> <span class="n">String</span> <span class="n">attrName</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">Assert</span><span class="o">.</span><span class="na">notNull</span><span class="o">(</span><span class="n">sc</span><span class="o">,</span> <span class="s">&#34;ServletContext must not be null&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 从 servletContext中获取父容器 WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">Object</span> <span class="n">attr</span> <span class="o">=</span> <span class="n">sc</span><span class="o">.</span><span class="na">getAttribute</span><span class="o">(</span><span class="n">attrName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 省略...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="k">return</span> <span class="o">(</span><span class="n">WebApplicationContext</span><span class="o">)</span> <span class="n">attr</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>我们在以XML的方式配置基础的springmvc项目时，在web.xml中通常有两个重要配置：ContextLoaderListener 和 DispatcherServlet。而上面的流程，就创建了这两个核心配置的内容。</p>
<p>如果我们不用XML方式配置，而是通过自定义一个 WebApplicationInitializer 实现类的方式，就可以做到无XML来启动基于springmvc的项目，所有的配置通过注解方式来完成。</p></div>

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